1. Field of the Invention
The present invention relates to a microstructure suitable for use in Raman spectrometry. The present invention also relates to a process for producing a microstructure suitable for use in Raman spectrometry. The present invention further relates to a process for performing Raman spectrometry and a Raman spectrometric system using the above microstructure.
2. Description of the Related Art
In Raman spectrometry, a material is irradiated with monochromatic excitation light, scattered light (called Raman scattered light) having wavelengths different from the wavelength of the excitation light is obtained, and a spectrum of the Raman scattered light (called Raman spectrum) is analyzed. The Raman scattered light has a very low intensity, and therefore detection of the Raman scattered light is generally not easy. However, it is reported that the intensity of the Raman scattered light is increased by a factor of 104 to 106 when the specimen (material to be analyzed) is absorbed by a metal surface before the irradiation. In particular, it is known that the intensity of the Raman scattered light is greatly increased when nanometer-size metal particles are dispersedly distributed over the surface by which the material to be analyzed is to be absorbed, as disclosed in “A complementary study of surface-enhanced Raman scattering and metal nanorod arrays”, J. L. Yao et al., Pure Appl. Chem., Vol. 72, No. 1, pp. 221-228 (2000). It is considered that the localized plasmon resonance increases the intensity of the Raman scattered light. That is, It is considered that free electrons in the nanometer-size metal particles vibrate in resonance with the electric field of the light, the vibration of the free electrons produces strong electric fields in the vicinities of the nanometer-size metal particles, and the strong electric fields increase the intensity of the Raman scattered light.
According to the process disclosed in the Yao reference, a device having a structure in which nanometer-size metal particles are dispersedly distributed is produced by forming an alumina layer by anodic oxidation of aluminum, and filling minute pores which are spontaneously formed at the surface of the alumina layer during the anodic oxidation, with metal. Specifically, after the minute pores are filled with the metal, the upper portion of the alumina layer is removed by etching so that upper portions of the minute metal particles protrude. Thus, the strong electric fields generated around the tips of the head portions of the minute metal particles increase the intensity of the Raman scattered light.
In the above structure, when the minute metal particles protrude higher, the intensity of the Raman scattered light is more increased. However, the time for which the alumina layer is etched exceeds a predetermined time, the head portions of the minute metal particles are broken down by the etching. Therefore, it is not easy to make the head portions of the minute metal particles protrude sufficiently high.